Fan with an essentially square housing

ABSTRACT

A fan assembly for the cooling of electronic systems, where a square housing is provided with a central electric motor for the impeller, and a first main inlet surface of the housing is arranged perpendicular to the axis of rotation of the impeller and in parallel to the inflow direction, and where the flow through the fan is deflected by 90° after it leaves the impeller to exit, the housing at at least one lateral outlet surface that is perpendicular to the first main inlet surface which assembly results in a drastic reduction of noise.

BACKGROUND OF THE INVENTION

The invention relates to a fan with an essentially square housing and animpeller that is centrally driven by an electric motor; with the axis ofrotation of the impeller being perpendicular to a first main inletsurface of the housing and in parallel to the inflow direction. The flowof air leaving the impeller being deflected by 90° leaving the fanhousing at at least one lateral surface of the housing that isperpendicular to said first main surface; and wherein a bottom surfaceof said housing that is opposite the inlet surface being developed as aclosed wall with the blade edges of the impeller on the outlet sidebeing spaced a distance away from the bottom surface.

Initially fans were equipped with so-called radial impellers; i.e., theair is deflected in the impeller itself from the intake direction by 90°into the outlet plane. This results in a higher pressure yield than bymeans of the so-called axial impeller fans. Fans of this type are knownfrom the German Published Patent Application 22 57 509 (DE-413). Similarfans are also known from DE-OS 21 39 036 (DE-409). In both cases, aconventional radial impeller was used in which the 90° deflection of theflow takes place inside the impeller.

However, solutions of this type (conversions of axial to radial flow)are also known where a deflection of the flow takes place in the area ofthe impeller itself although the shape of the impeller is that of anaxial wheel.

Thus, it is stated in DE-AS 15 03 609 that the delivered medium isalready subjected to a deflection in the first part of the impellerwheel and leaves the impeller wheel with a radial flow component.According to the objective that is described there, this solution seemsto be useful mainly for very high pressure requirements. This priorsolution also has a housing ring that expands conically in the directionof the flow delivery and extends approximately to over half the axialwidth of the impeller wheel. Because of this lack of complete coveringof the axial width, the solution permits the radial flow component inthe area of the impeller wheel. As far as the reduction of noise isconcerned, this solution is still very imperfect.

Another previously known solution according to DE-OS 18 02 523, like thelast-described arrangement, as far as the outward appearance isconcerned, shows an axial impeller, but here also, the ring thatsurrounds the impeller extends only to the axial center of the impeller,so that a deflection of the air in radial direction takes place insidethe impeller. In axial view, this arrangement is very large.

DE-PS 634 449 shows a spiral housing where the deflection of the airflow in radial direction takes place by means of very rounded blades intheir central area. The impeller that is used here is also an axialwheel, but the blades themselves deliver air radially beyond their outeredges into the flow space-analogously to the two last-describedsolutions. The tube that extends from an inlet plane into the axialcenter of the blades and encloses it is tapered extensively in flowdirection.

In all these previously known solutions, the blades have the function todeliver extensively in radial direction via their radially exteriorblade edges, and the deflection of the air takes place, as in the caseof the conventional radial impeller, inside said impeller. Thesesolutions are not suited to sufficiently satisfy today's predominantobjective of low noise while still retaining an axially compact fan.

In the electronics industry or in the data-processing industry, it isalso common to use fans of this type in connection with larger housingboxes for the ventilating of the electronic system located in theapparatus. It is increasingly required in these cases that the noiselevel be low, particularly in the field of miniature fans havingimpeller diameters of less than 200 mm. In practice, the situationexists that more compromises can be made with respect to the pressure orvolume per time, while very strict requirements exist with respect tonoise levels. The result is that frequently fans of this type areoperated at lower rotational speeds only for noise reasons. Thus, theconstant demand with respect to a "noise minimization" is a predominantaspect in the development of fans of this type.

Within the scope of this objective, it was surprisingly found that a fanwith an essentially square housing and an impeller that is centrallydriven by an electric motor; with the axis of rotation of the impellerbeing perpendicular to a first main inlet surface of the housing and inparallel to the inflow direction. The flow of air leaving the impellerbeing deflected by 90° leaving the fan housing at at least one lateralsurface of the housing that is perpendicular to said first main surface;and wherein a bottom surface of said housing that is opposite the inletsurface being developed as a closed wall with the blade edges of theimpeller on the outlet side being spaced a distance away from the bottomsurface is effective in its performance and extremely low in noise.

Thus, it was found, for example, that a fan that is constructedaccording to the state of the art, with rectangular parallelepipeddimensions of approximately 130×130×40 mm and was equipped with aconventional radial impeller, with respect to noise, was reduced to 44dba by means of special measures, whereas the fan according to theinvention, with the same dimensions and equipped with an essentiallysquare housing and an impeller that is centrally driven by an electricmotor; with the axis of rotation of the impeller being perpendicular toa first main inlet surface of the housing and in parallel to the inflowdirection. The flow of air leaving the impeller being deflected by 90°leaving the fan housing at at least one lateral surface of the housingthat is perpendicular to said first main surface; and wherein a bottomsurface of said housing that is opposite the inlet surface beingdeveloped as a closed wall with the blade edges of the impeller on theoutlet side being spaced a distance away from the bottom surface, andwherein the impeller is an axial impeller of the type that has anair-guiding outlet duct that is formed by a wall that radially on theoutside completely surrounds the blades and where the air flow leavesthe outlet edges of the impeller only in an axial direction, reducedthis value to 38 dba. (This applies to both embodiments.) Naturally,comparable pressure and volume capacities exist in each case. Thus, inthese operating cases, the pressure is relatively low and the volume ismoderate, thus, in the case of the characteric pressure-volume curve,mainly in the medium range, at least on the right of the salientstability point of the characteristic pressure-volume curve, air flowhas not yet "broken off".

Other advantageous developments are found with an essentially squarehousing and an impeller that is centrally driven by an electric motor;with the axis of rotation of the impeller being perpendicular to a firstmain inlet surface of the housing and in parallel to the inflowdirection. The flow of air leaving the impeller being deflected by 90°leaving the fan housing at at least one lateral surface of the housingthat is perpendicular to said first main surface; and wherein a bottomsurface of said housing that is opposite the inlet surface beingdeveloped as a closed wall with the blade edges of the impeller on theoutlet side being spaced a distance away from the bottom surface. Theimpeller is an axial impeller of the type that has an air-guiding outletduct that is formed by a wall that radially on the outside completelysurrounds the blades and where the air flow leaves the outlet edges ofthe impeller only in axial direction and has the impeller diameterapproximately 20% or 30% smaller than the outer side dimensions of arectangular parallelepiped housing. Also advantageous is having theimpeller with its blade edges that are located on the inlet side,disposed in the area of the air inlet plane as well as having theimpeller blade edges of the axial impeller located on the outlet side,disposed approximately in the center of the axial height of the fan. Itwas also found advantageous to have, at the area of the air inlet bladeedges of the axial impeller, a housing radially directly outside theimpeller, with rounding at its inlet, as well as in the area of theoutlet blade edges of the axial impeller, having the housing have arounding, radially outside the impeller, so that the air after leavingthe axial impeller at first encounters an enlarged flow cross-section.This enlarged flow cross-section can be caused by a diameter that is atleast about 10% larger than the outlet and is formed over the wholecircumference thereof. Another advantage of the invention is to have theblades of the impeller extend at least over half the axial height of thefan. Still further, it was advantageous to have the impeller blades takeup, one half to one third of the axial height of the fan, with an outletof the fan extending from the end of the impeller to the bottom wall andamounting to one half to one third of the axial height of the fan. Alsoit is advantageous to have the height of the housing be about 1/3 of theimpeller diameter. Additionally, it was found advantageous to have theinterior surface surrounding wall of the housing defining the outlet ofthe fan to be an essentially cylindrical flow ring.

Probably, the advantageous effect can be expected not only in the caseof a miniature fan of the type described in the following, but basciallyalso in the case of a larger construction. However, surprisingly, atleast in the case of this miniature size, the combination according tothe invention has proven to be extremely effective with respect to aminimizing of noise.

These and other objects, features, and advantages of the presentinvention will become more apparent from the following description whentaken in connection with the accompanying drawing which show, for thepurposes of illustration only, plural embodiments in accordance with thepresent invention, and wherein the figures show two embodiments of theinvention.

FIGS. 1 to 3 show a first embodiment.

FIG. 1 is a view from above and

FIG. 2 is a side view in partial section according to the cutting lineII/II of FIG. 3.

FIG. 3 is a view from below of a square housing block in which animpeller is arranged concentrically.

A second embodiment is shown in FIGS. 4 to 9.

FIG. 4 is a partial sectional view according to the cutting line IV/IVof FIG. 7 (similar to FIG. 2) of a complete fan according to the secondembodiment of invention;

FIG. 5 is a bottom sectional view of a component of FIG. 4 according tothe cutting line V/V of FIG. 6;

FIG. 6 is a bottom view of this component according to the Arrow VI ofFIG. 5;

FIG. 7 is a bottom view of the fan according to the Arrows VII in FIG.4, with the base plate removed along with the motor and the impellerbeing fastened to it.

FIG. 8 is a sectional representation according to the cutting lineVIII/VIII in FIG. 7; and

FIG. 9 is a side sectional view according to the cutting lines IX/IX ofFIG. 7;

FIG. 10 shows a graph plotting pressure rise across the fan (vertically)and flow volume (horizontally) for the operation of both embodiments bymeans of their pertaining operating points AP1 and AP2.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to the drawings wherein like reference numerals are usedto designate like parts and more particularly to FIGS. 1 to 3 which showa first embodiment. In this case, FIG. 1 is a top view of the air inletplane 7; FIG. 2 is a side view according to the Arrows II in FIGS. 1 and3, showing the outlet opening 32, and FIG. 3 is a bottom view of theclosed second main surface or bottom rear wall 6 of the housing. Theright-hand portion of FIG. 2 shows a partial sectional view according tothe cutting line II--II of FIG. 3. FIG. 1 and FIG. 2 show a centraldriving motor 8 which advantageously is developed as a so-calledexternal rotor motor. In this case, it carries five axial flow blades 9that are tilted by about 45° and are slightly bent. If the motor is anexternal rotor, the impeller advantageously is a one-piece plastic parthaving a cup-shaped hub that is put in an inverted position over themotor, and plastic blades 9 are integrally injection-molded onto it. Thedriving motor, that is located inside the impeller hub 8, via screwelements 25, 26, is fastened by means of its stator from the directionof the closed base plate 6. The internal stator, that is disposed underthe impeller hub 8, is fastened via the flange part 28; and via theplate 29, the whole impeller with the rotor is fastened so that it isalso rotatably disposed. The outflow direction of the fan is marked bythe arrows W. The air inlet plane 7 ends with the housing top of the.The head of the impeller hub and the blade edges 21 on the inlet sideare also located in this plane. Radial outwards of the blade 9 is acylindrical member 39 which has a round inlet part 12 located at theinlet side of the fan facing the blades. Similar structure is shown inthe embodiment of FIG. 4.

As shown in FIG. 1, the air guiding duct around the blades 9 is acylinder 39 with the inside diameter 27. In the case of one successfulembodiment, it measures 115 mm. The pertaining impeller 9 diameter 24measures about 112 to 113 mm. This means that there is an air gap of 1mm radially on the outside, between the blades and the surrounding wall.That is still acceptable with respect to the flow quality andmanufacturing expenditures. The smaller the gap, the better is the flow,but more expensive is the manufacturing.

The walls 2, 3, 4 are closed lateral surfaces, while the lateral surface5 is open. The air in the area of the axial height 32 flows laterallyout through the lateral surface 5. In this lower area 32, only thestator with the flange 28 is disposed, and the fastening element 29 isprovided centrally in the area of the motor. In the successfulembodiment, the measurement of the partial axial height 32 is 17 mm,while the upper partial axial measurement 31 is 22 mm. The exhaustopening 32 in the plane of the lateral surface 5 therefore starts in thearea below the blade 9 whereat side edge 19, as shown below the topportion of the lateral surface 5 in FIG. 2. FIG. 2 is therefore apartial sectional view. The upper portion of surface 5, that in thepartial sectional view of FIG. 2 shows the wall ring 39 with the roundededges 12, 13 on the inlet side and outlet side, each having a radius ofcurvature of about 5 mm in the embodiment, circumferentially surroundsthe blades. On the outlet side of the housing, namely the lateralsurface 5, barely half of this lateral surface is open at 32, for use asthe outlet. The closed interior wall surfaces 2, 3, 4 are recessedoutwardly from the flow wall ring 39 (having an inside diameter 27) by acertain amount so that the flow after leaving the impeller 9 in an axialdirection (downwardly in FIG. 2) can at first still open up into aslightly larger cross-section defined by the side wall dimensions 22.However, it is advantageous for the corner areas in the housing belowthe wall ring 39 and between the rectangular inner surfaces of the walls2, 3 and 4, to extend from the center of the wall 3 to the center of thewall 2 and to the center of the wall 4 in a rounded out circular mannersuch that the distance between this interior circular wall surfaceconnecting the centers of the plane walls 2, 3 and 4 is approximatelyequal to the inside diameter 27 of the wall ring 39; i.e., the wall,from one center to the other, in a way that is not shown in FIGS. 1 to3, is rounded out in a circular shape, in which case the center of thecircle is the axis of rotation of the fan.

The full axial impeller dimension of 22 mm (seen in axial flowdirection) is therefore located behind the closed area of the lateralsurface 5 with the height 31.

For the purpose of minimizing noise according to the invention, normalaxial impellers that are axially compact can be inserted into a housingthat corresponds to the invention. A favorable ratio will then beobtained between a low but still quite useful pressure and volume andthe noise values. It is important that the preferably cylindrical flowtube (39) axially surrounds the impeller completely.

It should be pointed out that fans of this type are standardized in allouter dimensions and therefore have maximum dimensions. Within thesedimensions, different fans having an optimum of noise reduction andrequired capacity or pressure must be achieved. Thus various fan sizesand configurations are used to fit into this standardized outerdimension. Because of the fact that the fan can be fastened, in thesimple way shown in FIG. 2, is possible to mount various fans directlyto the rear wall 6 because the axial impeller with a stator can bepractically extended via the elements 28, 29, 25, 26 The use of fanhousings of this type, for conventional radial impellers is stillpossible.

FIGS. 1 to 3 show the first embodiment in half its natural size.

In FIGS. 4 to 9, the same reference numbers as in FIGS. 1 to 3 are usedfor the parts that have the same function while corresponding parts areprefaced by the numeral 100.

In the case of the second embodiment according to FIG. 4 to 9, theimpeller diameter 124 is slightly more reduced with respect to the outeroverall dimension 112 of the housing amounting to approximately 67% ofthe dimension 122. The rotational speed (about 2,300 rpm) of thissmaller axial impeller is higher than the rotational speed (about 2,000rpm) of the impeller according to FIGS. 1 to 3 of the first embodiment,the diameter of which is larger (amounting to approximately 83% of themeasurements of the housing 22). The second embodiment meets the demandswith respect to noise reduction very well, irrespective of the fact thatthe pressure requirement is twice as high compared to the firstembodiment. The eccentric positioning of the impeller 9 in the housingthat is used in the second embodiment is known per se and still resultsin a certain improvement of the air output while the noise remains low.

In FIG. 10, the characteristic resistance curves AW1 and AW2 are enteredby interrupted lines for two certain applications. AX1 is thecharacteristic fan curve for the first embodiment. The operating pointAP1 may also, as previously, be operated by means of a radial fan wheelaccording to the characteristic curve RL. However, in that case, thenoise would be much too high. If, also within the scope of the presentinvention, the axial wheel according to the first embodiment is operatedin a fan of this type with an increased rotational speed, then thecharacteristic apparatus line AX1' would apply, with the operating pointAP2 of the characteristic resistance line AW2 being attained in thisway. However, it was found that for this application, an arrangementaccording to the invention that is constructed according to embodiment 2is better. The characteristic curve AX2 corresponds to this secondembodiment, and with a further reduced impeller diameter and with aslightly higher rotational speed, despite the inreased pressurerequirement, a very good noise behavior is still achieved (compare abovevalues). In the case of this comparison, the outer dimensions of therectangular parallelepiped housing are practically the same.

Similar to FIG. 2, FIG. 4 is a partial sectional view through a completefan according to the second embodiment. In this case, similar to whatwas described in the German Patent Text 22 57 509, the fan housing isdeveloped as a one-piece cup-shaped plastic part having the walls 2, 4,front plate 70 and flow ring 39, and is screwed against the bottom plate6 that is developed as a simple punched bent component. On said bottomplate 6, the whole impeller is mounted with the coaxial, concentric,driving electric motor that is an external rotor motor, as in FIG. 2, bymeans of screws 25, 26 is attached against a conically indented circularfastening plate 129 that is pressed out of the bottom plate 6 and has aspace 62 with respect to the bottom plane 6 (see FIG. 5). The distance62 is maintained in such a way that it corresponds to the optimal axialposition of the existing fan wheel 8, 9. The internal stator of theexteral rotor motor has a flange plate 28 that is developed in one piecewith the inner bearing support pipe element 128 of the driving motor, sothat the screws 25, 26 simply reach through the openings 25', 26' of thefastening plate 129 into theads of the flange ring 28, in which case theheads of the screws 25, 26 are located in the conical indentation.

The left side of FIG. 4, in a drawn-out way, shows the optimal axialposition of the impeller, in which case the blade edges 21 on the inletside are provided close to the inflow plane 7, but still in the area ofthe inlet rounding 12 edge of flow ring 39 and with, the blade edges 19on the outlet side axially ending with the bottom edge 40 of the flowpipe 39.

The right side of FIG. 4 shows a somewhat less advantageous positionwhich however is somewhat better for inflow conditions, because theedges 21 on the inlet side of the blades axially connect to the lowpoint of inflow rounding edge 12 of the flow pipe 39. However, accordingto the embodiments of the invention, the impeller with its blades 9should project approximately no further axially beyond the bottom edge40 of the flow pipe 39 than is shown in the right part of FIG. 4, namelywith the blades edges 19, 29 on the outlet side, no more than 2 mm orabout 10% of the axial blade length below bottom edge 40. If the bladeedge 29 on the outlet side is spaced axially further away from the end40 of the flow pipe 39, the noise will be increased considerably.

FIG. 6 is a complete top view of the base plate 6, in which case, asmentioned above, screws 25, 26 for the mounting of the flange 28 of themotor reach through the openings 25', 26' of the circular, conicallyindented fastening plate 129 to which the base plate connects via aconical intermediate portion 67.

While FIGS. 4 to 6 represent the actual size of the second embodiment,FIGS. 7 to 9, for reasons of representation, are reduced. The axis ofrotation 100, in the base plate of FIG. 6 as well as in FIG. 7,indicates the position of the impeller 8, 9 in the housing 6, 77. Theeccentric offsetting is known per se, for example, from DE-PS 21 39 036,in which case the distance between the housing walls increases in flowdirection. Thus in FIG. 7, the lengths 112, 113, 114 of the distancesare characterized by the lengths of their arrows, which increase betweenthe flow ring 39 and the round wall 139. The distances according tonumbers 112, 113, 114 are approximately on the order of 1 to 3 to 3, inwhich case, on the outlet side, the outer round wall 139 was left outover the whole width 120 of the outlet cross-section.

In the case of the first embodiment, the outlet area on the lateralsurface 5 is limited to the distance 32 between the flow ring 39 and thebase plate 6, but in the case of the second embodiment, the outlet areaextends over the full axial height 121 of the housing for the freeoutlet cross-section. However, the outlet flow under the edge 40 iscertainly stronger in the area of the base plate 6. Whether the outletheight 121 is utilized only over a part (for example, part 32 of thehousing height 33) or completely (at 133), is of only subordinatesignificance.

FIG. 7 shows a top view of the cup-shaped plastic housing from thebottom which housing is screwed against the base plate 6 according toFIGS. 5/6 which is not shown in FIG. 1. At the lower edge 45 of theopposing lateral walls 4, 2, as shown in FIG. 4, a surrounding shoulder44 is provided above the circumference, into which the metallic baseplate 6 engages in a form-locking manner, before it is screwed togetherwith the plastic holding shell 77 via the bolt-type elements 71 to 74that are injection-molded to it. In FIG. 7, the head surface 45 of theshoulder 44 that is practically in alignment with the exterior wall ofthe base plate 6 is drawn in black.

FIG. 7 shows the eccentric position of the impeller axis 100 in thehousing. The axis 101 that is located symmetrically in the housing 77has practically the same distance from the outer walls 2, 4 thatcorresponds to the radius 111 of the round wall 139 of the exhaust duct.The latter extends as a semicircle between the lateral walls 2, 4. InFIG. 7, the axis 100 is shown offset in two directions (a and b)counterclockwise from the direction of the axis of symmetry 101 (likethe rotating direction of the impeller that is indicated by aninterrupted line by means of the Arrow 107).

The first step (a) in outlet direction and the second step (b)subsequently to the left of the outlet direction each has a length ofabout 10% of the length of the radius 111. The round wall 139 extendsaxially from the top front plate 70 completely to the bottom plate 6,whereas the flow pipe 39 with its edge 40 terminates at a distance tothe bottom plate 6.

While we have shown and described only plural embodiments in accordancewith the present invention, it is understood that the same is notlimited thereto but is susceptible to numerous changes and modificationsas known to one having ordinary skill in the art, and we therefore donot wish to be limited to the details shown and described herein, butintend to cover all such modifications as are encompassed by the scopeof the appended claims.

We claim:
 1. A fan assembly having a rectangular parallelepiped housingand an impeller means having blade means and said impeller means beingcentrally driven by an electric motor; wherein an axis of rotation ofthe impeller means is perpendicular to a first main inlet surface of thehousing and parallel to a flow through the fan; the flow through the fanbeing deflected by 90° and leaving the housing through at least onelateral surface of the housing that is perpendicular to said first maininlet surface; a second main closed wall surface in said housing that isopposite the first main inlet surface; outlet blade edges on the blademeans are spaced downstream in a flow direction from the air inletsurface and spaced from the second main closed wall surface; theimpeller means producing an axial flow through the blade means andhaving an air-guiding duct, that is formed by a wall means that radiallyand completely surrounds the blades means to cause flow through the fanto exit the outlet blade edges only in an axial direction.
 2. A fanassembly according to claim 1, wherein an outside diameter of theimpeller means is approximately 20% smaller than a dimension of therectangular parallelepiped housing, perpendicular to the impellerrotation axis.
 3. A fan assembly according to claim 2, wherein theimpeller means has inlet blade edges means that are disposed in the areaof the first main inlet surface plane.
 4. A fan assembly according toclaim 2, wherein the impeller outlet blade edge means are disposedapproximately in the center of an axial height of the rectangularparallelepiped housing.
 5. A fan assembly according to claim 2, whereinin the area of the air inlet blade edge means, the air guiding ductmeans radially and directly outside the blade means has a rounded inlet.6. A fan assembly according to claim 1, wherein an outside diameter ofthe impeller means is approximately 30% smaller than a dimension of therectangular parallelepiped housing, perpendicular to the impellerrotation axis.
 7. A fan assembly according to claim 1, wherein theimpeller means has inlet blade edges means that are disposed in the areaof the first main inlet surface plane.
 8. A fan assembly according toclaim 7, wherein the impeller outlet blade edge means are disposedapproximately in the center of an axial height of the rectangularparallelepiped housing.
 9. A fan assembly according to claim 7, whereinin the area of the air inlet blade edge means, the air guiding ductmeans radially and directly outside the blade means has a rounded inlet.10. A fan assembly according to claim 1, wherein the impeller outletblade edge means are disposed approximately in the center of an axialheight of the rectangular parallelepiped housing.
 11. A fan assemblyaccording to claim 10, wherein in the area of the air inlet blade edgemeans, the air guiding duct means radially and directly outside theblade means has a rounded inlet.
 12. A fan assembly according to claim11, wherein in the area of the outlet blade edge means, the air guidingduct means radially and directly outside the blade means, has a roundedoutlet part so that the flow leaving the axial impeller, at firstencounters an enlarged flow cross-section.
 13. A fan assembly accordingto claim 12, wherein the flow after leaving the rounded outlet partpasses to an enlarged flow cross-section, defined by a diameter memberthat is formed over a whole circumference of the housing means and is atleast about 10% larger than an outside diameter of the impeller means.14. A fan assembly according to claim 1, wherein the impeller meansextends in the flow direction for at least over half the axial height ofthe housing.
 15. A fan assembly according to claim 14, wherein theimpeller means extends from at least one half to one third of the axialheight of the housing, and wherein a flow outlet opening in the housingis located between the outlet edge means of the blade means and theclosed wall surface and has a height of one half to one third of theaxial height of the housing.
 16. A fan assembly according to claim 1,wherein the distance between the main inlet surface and the closed wallsurface is equal to about 1/3 of the diameter of the impeller means. 17.A fan assembly according to claim 1, wherein the surrounding wall of theair guiding duct means is configured as an essentially cylindrical flowring.
 18. A fan assembly according to claim 1, wherein in the area ofthe air inlet blade edge means, the air guiding duct means radially anddirectly outside the blade means has a rounded inlet.